It is generally known to provide key arrays in the form of keyboards or of rows of keys, in which each key has associated with it an actuating device such as a switch contact which is closed to perform a switching function in response to an actuation of the respective key, or delivers a switching signal to a utilizing circuit as soon as the associated key is depressed. Known key arrays of this kind can be reduced in size only within certain limits because each key must be so large that an operator's fingertip can safely engage the key without depressing an adjacent key, possibly initiating an undesired function. To avoid an undesired actuation of keys where the keys are relatively small, it is known to provide an actuating pin or stylus for depressing individual keys, for example, in midget computers. Alternatively, the individual keys may be formed like islands having a raised mushroom-shaped surface for engagement by the actuating finger so that an adjacent key cannot be depressed together with or instead of the desired key unless the desired key is highly eccentrically engaged by the actuating finger.
In numerous cases it is desired to provide keys having a large legend surface for visual clarity and to enable a convenient actuation of the keys in that the key surface which is engageable by the actuating finger is at least as large as the projection of the fingertip. With reduced size keyboards this cannot be accomplished with the known key arrays because the keys which are provided in midget key arrays and can be depressed only with an actuating stylus cannot be provided with clearly visible legends. Further, a separate element is required for their actuation. In addition, problems arise because the island type keys present also only a greatly restricted legend surface and create a visual impression which often confuses the operator. Also, if the top of the key is sufficiently small for the desired purpose, the key cannot be actuated conveniently. It is further known from U.S. Pat. No. 2,261,115 (Hofgaard) to reduce the number of keys necessary for initiating a predetermined number of switching functions or switching signal combinations. This is accomplished by providing key groups in different levels and of different shape so that the fingertip of an operator is enabled to perform an encoding operation by which selectively either the respective keys singularly or together with one or two neighboring keys can be depressed. The legends or symbols are correspondingly provided on the surfaces belonging to one single key and on surfaces common to a plurality of keys, respectively. It is a disadvantage of the Hofgaard key arrangement that, at least with regard to some of the symbols and corresponding signal combinations, the key surface available for a safe and unambiguous operation and for providing a legible legend is not large enough.
Another device having a compound key/signal arrangement is shown in U.S. Pat. No. 2,730,248 (Van Waert). In this keyboard, output signals are obtained by depressing one key, a combination of two keys, or a combination of four adjacent keys. Each key has a function whether individually or in combination with another one or three keys which are depressed simultaneously.